I would really give other OS(32bit)/machine a chance.

If you did already im sorry for repeating myself, but trying all those amounts of failed flash in same environment aren't taking you anywhere...


If it was me i would:

1st use other os 32bit/machine

2nd Keep up with the battery. Try new battery? even other and shorter USB cable.

Maybe (just a maybe) your N900 board as contaminants in it that may prevent it to function normally, if not really damaged as many happen to be at some point.

3rd open it up (carefully, very easy, service manual and youtube tuts available wildly to do it) and clean it up with isopropyl alcohol or good pcb cleaner. After dry (10 minutes, not so long but...) assemble it back and try with the flash again.

But hey, this is just me ok? If some here (more experienced for sure than me on this handset) tells you not to follow my tips, don't follow my tips please.